In the above preferred applications, the shaped parts that are to be obtained require the use of a shaping mold of the type comprising a punch and a die, serving to shape a blank of material based on thermoplastic composites, which blank is subjected to a preheating stage prior to being engaged in the mold, thereby imparting an appropriate temperature to the material for subsequent shaping of its thickness by means of the mold which is itself cooled for this purpose, which shaping generally includes compacting or compression of the material.
Blanks for constituting individual parts are cut to an appropriate plane shape and are generally selected to be made up of padding materials which may be woven or non-woven, including greater or smaller quantities of fibers or threads of natural or synthetic or thermoplastic material, such as polypropylene or polyethylene. Any base material can thus be appropriate, and in this respect it is general practice to use layers of woven or non-woven or felted material.
It should naturally be understood that in the meaning of the invention, the particular nature of the blanks is part of the prior art that is known and available to any person skilled in the art.
The blanks, which in some cases can be thought of as stiff sheets, are thus heated, generally right through, either by infrared radiation or by applying hot air, or indeed by a so-called "contact" method, it also being possible to combine these various methods.
When heating blanks or sheets by contact, it is appropriate to use a kind of press in which hot plates are brought with controlled relative pressure into contact with the two large faces of each blank or sheet in order to transfer sufficient heat to the component fibers thereof for softening the thermoplastic material.
To be successful, such a method generally requires successive temperature-raising steps to be performed in order to reduce the cycle type required for transformation and thus increase production throughput. That is why such presses generally include two or even more contact heating press assemblies, each assembly implementing respective pressures and temperatures that may be at different levels.
That technique is likewise known to the person skilled in the art.
Nevertheless, in an application of the kind outlined above, a problem that arises is that of inserting and transferring blanks from one heating assembly to another, and to an even greater extent, of extracting the blank from the last heating assembly in order to bring each blank into the mold proper by means of a pickup transporter.
The difficulty arises because of the sticky character imparted to the two large faces of the blank, and also because of its softened state which does not help in providing suitable conditions for such a blank to be picked up successfully, given its adhesive character.
In an attempt to solve this technical difficulty, it has been recommended to make use of extractor clamps that may be situated transversely to the travel direction of the blanks, or parallel to said direction.
Said clamps must initially take hold of the edge or margin of each blank and exert sufficient pressure on the material thereof to cause it to move relative to a moving transporter or a static conveyor, in spite of the sticky or adhesive character of the blank.
Such extractor clamps must therefore act on material that is strong enough and preferably not sticky, which is why such blanks are initially cut to a shape leaving excess margins which are not heated by contact and which are subsequently cut off.
Operating in that way subjects the shaping operation to a severe cost penalty and it is generally the subcontractor who must bear the cost of the additional raw material which is scrapped.
Another drawback of the technique using pickup clamps stems from the fact that they are not adapted to taking hold of blanks whose edges to be grasped are not parallel to the direction in which the clamps are set up. Unfortunately, this situation arises frequently, given that the blanks need to have plane shapes that correspond to the shapes of the parts which are to be obtained, for legitimate reasons of not wasting material. This applies to plane shapes in the form of non-rectangular quadrilaterals.
Account should also be taken of prior art proposals which consist in implementing conveyor belts made of materials that have the reputation of being unaffected by adhesive or sticky materials. In this respect, mention can be made of belts made of glass fibers that are coated or covered in an appropriate material, such as polytetrafluoroethylene.
In practice, those proposals have not been found to be entirely satisfactory since it often happens that when a hot blank is extracted from a press on its way to the mold, there is uncontrolled adhesion of the blank which remains stuck to the conveyor belt, regardless of whether the belt is constituted by a single endless belt or by two endless belts with facing lengths. Under such circumstances, a blank which sticks to one or other of the belts is then brought back to the inlet of the pressing and heating station, which can therefore no longer operate automatically.
The running of the entire installation is then seriously disturbed, giving rise to reduced throughput, and indeed the need to stop operation, e.g. when the uncontrolled and random movement of a blank clogs up an operating subassembly which needs to be put back into its initial state before the machine as a whole can operate properly. It is clear that such situations make it difficult to achieve automatic operation, even though that is preferred for such machines.
Finally, mention should be made of another prior art proposal consisting in taking each blank at the entrance to the pressing and heating station, sandwiching it between two fine pieces of cloth which may be woven or non-woven, and which are provided to oppose adhesion. That solution achieves its result, but it increases production costs in a non-negligible manner.